This invention relates to an apparatus and a method for pulling down the voltage of a word line in a memory.
This invention relates specifically to an apparatus and a method which steers a pull-down current to the word line in response to a signal that a word is to be deaddressed and which is adaptive and self-regulating in that the pull-down current is steered to the word line only as long as the pull-down current is needed to pull the word down.
One conventional form of memory is organized in words and columns. Each word of the memory is associated with a top word line and a bottom word line, and the voltages on the word lines for a particular word must be pulled down from high to low when a word is deaddressed.
The general way of accomplishing the pull down is with a diode coupled pull-down arrangement in which the pull-down current is shared by more than one word.
The primary defect with the prior art is that when a word is being deaddressed, the pull-down current is steered away from that word before the word is fully deaddressed. The plot of the voltage as a function of time shows that the word falls rapidly, then as the word being addressed picks up the pull-down current, the rate of fall of the word being deaddressed decreases. This places an intrinsic lower limit of the cycle time of the memory.
The total current available to remove stored charge in the memory cells is the sum of the cell standby current and the pull-down current. As memory cells evolve, the standby current becomes smaller. Therefore, large pull-down currents are required. In the prior art diode coupled pull down it is required that the addressed word driver supply the static pull-down current. This in turn necessitates the use of large transistors and wide metal lines to ensure long term reliability of operation.